Actively sound reduced muffler having a venturi effect configuration

ABSTRACT

In an actively sound dampened muffler, the improvement of a venturi effect area producing means adjacent an aperture in said muffler near which is mounted a loudspeaker in an enclosure, the venturi effect area preventing hot exhaust gases from impinging on the loudspeaker.

In the design of active noise controlled mufflers a common problem isthe resulting lengthy extension which protrudes beyond the mainenclosure to the end of the vehicle resulting in an unattractive"snout". Beyond the physical undesirability of this extension there is amore serious problem created and that is the large acoustic air massthat resides in the snout. As this air mass is extremely difficult tomove to and fro at high frequencies the result is a small amount ofanti-sound being generated.

To overcome this problem it is proposed to keep the extension or snoutshort which reduces the moving mass. The solution has typically resultedin a configuration that has a very short link between the loudspeakerand the pipe and wherein the loudspeaker is typically placed far fromthe engine for the benefit of reduced sound pressure and temperature.The controller can then cancel the exhaust noise provided only oneacoustic mode can propagate in the exhaust pipe over the controlbandwidth. The loudspeaker enclosure volume may be reduced somewhat andthe diameter of the tail-pipe need not be larger than normal. Theproblems with this solution were that the loudspeaker is subjected tohigh temperatures as well as the corrosive effects of exhaust gases.

To solve these residual problems the instant invention manipulates themean flow of the exhaust gases to cool the loudspeaker enclosure. Eithera full or partial venturi is used to generate an area that is belowatmospheric pressure. A bleed pipe allows ambient air to flow into thisarea thereby counteracting the flow of exhaust gases into the speakerarea. A filter can be provided in the bleed pipe to reduce the radiatednoise. Additionally a heat shield can be employed to protect theloudspeaker from the exhaust pipe or, instead, a reflective coated layerthat conducts sound. To prevent a sudden reversal of the cooling air theconfiguration is designed to allow the cooling flow to maintain theexhaust gas concentration in the front cavity at a reasonable level. Adiffuser section can be used to accomplish this as well as a contractedsection followed immediately by an expansion section. Cooling can alsobe accomplished by using the cooling fluid of the vehicle or by usingseparate fans. Both would entail additional expenditures and add adegree of complexity to the configuration.

Therefore, it is an object of this invention to provide an improvedmuffler that eliminates the long loudspeaker enclosure.

Another object of this invention is to provide an actively controllednoiseless muffler having a short connection between the exhaust pipe andthe loudspeaker enclosure.

A further object of this invention is to an active noise cancellationsystem for an exhaust system wherein the diameter/size of the exhaustpipe can be of standard size.

A still further object of this invention is to provide an activelycontrolled muffler configuration that prevents ingestion of exhaustgases into the loudspeaker enclosure.

Yet another object of this invention is the provision of a venturi meansin an active noise cancellation exhaust system to create a differentialpressure adjacent the speaker enclosure.

These and other objects of this invention not specifically enumeratedherein will become apparent as reference is made to the followingdetailed discussion and drawings in which

FIG. 1 is a diagrammatic view of a standard active noise cancellationmuffler, and

FIG. 2 is a diagrammatic view of an improved configuration of animproved active noise cancellation muffler system and

FIG. 3 is a diagrammatic view of an in-pipe or venturi configuration inan improved active noise cancellation muffler, and

FIG. 4 is a diagrammatic view of a venturi nozzle configuration in anactive noise cancellation muffler to reduce radiation heating, and

FIG. 5 is another diagrammatic view of an in-pipe or venturiconfiguration in an improved active noise cancellation muffler showing aseparate heat shield, and

FIG. 6 is a diagrammatic view of a partial venturi nozzle configurationin an active noise cancellation muffler.

Referring to the figures, FIG. 1 shows a state-of-the-art active noisecancellation muffler having an exhaust pipe 1, a speaker enclosure 2containing a loudspeaker 3 and having an extension which is co-axial andlarger than pipe 1 and which also extends back beyond fender 5 andterminates in the same plane as pipe 1.

Demands on the performance of the muffler suggest that the loudspeakersmust function up to 500 Hz and this causes considerable problems in thedesign of the loudspeaker units for the following reasons:

(a) The current design of the active muffler requires the loudspeakerunits to be positioned in front of the rear fender, and yet the exitfrom the loudspeaker enclosure must be at the end of the muffler pipewhich is behind the fender.

(b) Inevitably, this leads to a "snout" on the enclosure that is severalinches long, and the acoustic mass of the air in the snout isconsiderable.

(c) At high frequencies it is difficult to move the slug of air in thesnout backwards and forwards sufficiently to generate much anti-sound.This requires a very small volume between the loudspeaker and the snout,and results in a a very large magnet assembly for the loudspeaker unit.

In order to avoid the large moving mass of the snout, it is essential tokeep the snout very short indeed, and this precludes the use of thecurrent design.

FIG. 2 shows an improved system wherein the noise is cancelled directlyin the muffler pipe. Exhaust pipe 11 is directly open to enclosure 12containing loudspeaker 13. There is no long extension or snout and theadvantages of this configuration are

(a) the link between the loudspeaker and the pipe can be very short,therefor reducing the moving mass;

(b) The active system can be placed at any convenient location in themuffler pipe (although far from the engine has the benefits of reducedsound pressure and temperature);

(c) provided only one acoustic mode can propagate in the exhaust pipeover the control bandwidth, the controller can in principle "cancel" theexhaust noise perfectly; this is not the case with the state of the artconfiguration because the source and anti-source are not quitecoincident;

(d) the diameter of the tail-pipe on the muffler need be little, if any,larger than it would be on a passive system;

(e) overall loudspeaker enclosure volume is reduced somewhat. The maindisadvantages of this improved configuration, however are:

(a) the high temperature that the loudspeaker unit might be subject to;

(b) ingestion of exhaust gases into the front cavity or the loudspeakerenclosure and consequent corrosion.

In order to circumvent these problems, the present invention manipulatesthe mean flow of the exhaust gases to force a clean, cooling airflowthrough the front (and/or back) cavities of the loudspeaker enclosure.One configuration is shown in FIG. 3.

An exhaust pipe 21 has a reduced section as at 22 so as to produce a lowpressure region at opening 23 into enclosure 24. Loudspeaker 25 ismounted in enclosure 24 so as to cancel, by counter-sound, noiseemanating at the exit 28.

The smooth contraction/expansion (a "venturi") 22 is used to generatelocally in the pipe a region that is below atmospheric pressure. A bleedpipe 26 in the front cavity of the loudspeaker enclosure 24 permits airfrom outside to flow towards the low pressure region, thereby reducingingestion of exhaust gases into the loudspeaker enclosure, and alsoproviding cooling.

By keeping the entrance 27 to the bleed pipe a little away from theexhaust exit 28, a cleaner flow will result, and help to reducecorrosion. On the other hand, some sound will escape from the bleedpipe, and it is advantageous to position the entrance of the bleed pipeclose to the residual error microphone 29 so that the control systemwill "cancel" the combined sound of the exhaust and bleed pipe noises.If pipe 26 is quite long, and not too large in diameter, radiation willnot be very strong and will be very frequency selective. The addition ofa filter in the bleed pipe could also help to reduce the radiated noise.When radiation from the bleed pipe is not a problem, its entrance can beplaced wherever is convenient.

Bleed-pipe routing should avoid water traps ("sumps"), and it may bepossible for the bleed pipe to drain the loudspeaker enclosure in theevent of flooding.

To avoid having the loudspeaker unit(s) face directly into the hole inthe muffler pipe since radiation of heat from the hot gasses could thendirectly heat the loudspeaker diaphragms (this may not be a seriousproblem as the emissivity of the gas is likely to be much lower thanthat of the pipe), the hole 42 in the muffler pipe 41 is arranged sothat the loudspeaker is shielded by the muffler pipe itself. See FIG. 4.A bleed pipe 45 conducts a cooling flow into speaker enclosure 44.

As shown in FIG. 5 a separate heat shield 52 can be used in conjunctionwith exhaust pipe 51 with its opening 53 across from loudspeaker 54 inspeaker enclosure 55. A cooling flow is brought in through cooling bleedpipe 56.

The heat shield can be a reflective coated substratum that isacoustically transparent (e.g. a coated "foam"; overlapping, separatedgrids etc.) and span most or all of the width of the front cavity, or itcan be a solid shield just large enough to "shadow" the diaphragm fromthe hot components without introducing significant acoustic blockage. Itcan be adequate just to have a reflective coating on heat-sensitiveparts such as the diaphragm.

The crucial aspects of this design are:

(a) generating the cooling flow with the minimum backpressure for theexhaust flow; and

(b) the detailed fluid mechanics of the flow through the section inorder to reduce turbulence, ingestion of gases into the loudspeakerenclosure (despite the cooling flow), flow-excited instabilities andflow noise.

Although on average the cooling flow will be flowing into the mufflerpipe, the strong oscillatory component in the muffler flow and theanti-sound may cause the flow to reverse instantaneously and ingestexhaust gases into the loudspeaker enclosure. This is not a problemprovided that on average the cooling flow keeps the exhaust gasconcentration in the front cavity at a reasonable level.

The classical venturi arrangement of a smooth contraction followed by adiffuser section is not the only way of producing a low pressure regionin the muffler flow (although it is a good one), for example acontraction followed by an abrupt expansion will also work, but withpotentially higher backpressure penalties. One example of an alternativeto the full venturi is sketched in FIG. 6. The principle of the idea isthat the mean flow of the exhaust gasses is manipulated in such a waythat an auxiliary cooling flow can be generated.

An exhaust pipe 61 is shown to have a partial venturi effect produced bysplit opening 62 which opens the pipe to speaker enclosure 63 containingloudspeaker 64 and bleed pipe 65.

Forced cooling of the loudspeakers by e.g. fans can obviously be done,but this would increase the complication and cost of the system. It isalso possible to use the car's forward motion to drive a cooling flowover the loudspeakers, but this would not work with the car stationary.Cooling with fluid (e.g. by tapping into the engine-cooling circuit)will also work, again at the cost of complexity, but it might bedifficult to get sufficient cooling on moving parts such as thediaphragms.

Having described several aspects of the invention it is anticipated thatother variations will occur to those of ordinary skill in the artwithout departing from the scope of the appended claims.

What is claimed is as follows:
 1. An actively noise canceled mufflersystem comprising:an exhaust pipe having a front end and a rear end, forexhausting exhaust gases; an aperture in the exhaust pipe disposedbetween said front end and said rear end of said exhaust pipe; anenclosure mounted adjacent said exhaust pipe and over said aperturecontaining at least one loudspeaker means adapted to produce oppositesound from that emanating from said exhaust pipe through said aperture;a venturi effect means in said exhaust pipe adjacent said aperture forgenerating a low pressure region in said exhaust pipe, said low pressureregion being below atmospheric pressure; a conduit means having a firstend and a second end, said first end being disposed in said enclosureadjacent said low pressure region and said second end being spaced fromsaid rear end of said exhaust pipe, for allowing ambient air to flow tosaid low pressure region to thereby prevent or inhibit any exhaust gasesfrom coming into contact with said loudspeaker means.
 2. A system as inclaim 1, further comprising a depressed cross-sectional area in saidexhaust pipe, said aperture contained in a section of said exhaust pipeopposite said depressed cross-sectional area.
 3. A system as in claim 1,wherein said venturi effect means is a reduced cross-sectional area ofsaid exhaust pipe.
 4. A system as in claim 1, wherein said conduit meansis a bleed pipe.